Fixed an issue reading symbolic values from hidden fields.

I don't understand why this change could make any difference to the execution, but seemingly the intermediate variable introduced in a34100a1e666eb652b3612641e37ba4bc142b7f4 has some side-effect. In the debugger I was also seeing some strange behaviour, where this variable would appear to be the empty string "" until I specifically inspected its value, at which point it would update to the correct value and execution would then continue as expected. Potentially could be some subtle timing bug maybe?

This should fix the hidden-field.html test case, which is part of issue #96.
Merge branch 'master' of github.com:cs-au-dk/Artemis
Fix typo in jquery-checkbox concolic test.